1. Field of the Invention
This invention relates to machinable organosiloxane elastomers. More particularly, this invention relates to organosiloxane compositions yielding erosion-resistant elastomers suitable for use as abradable seals between the moving and stationary elements of motors and turbine type compressors.
2. Background Information
The use of filled organosiloxane resins to form abradable seals in axial flow compressors of the type used in turbines for aircraft jet engines is taught in British Patent No. 791,568, which issued on Mar. 5, 1958. The purpose of the seal is to minimize the clearance between tips of the rotor blades and the housing of the compressor. The clearance is formed by applying a layer of curable resin on the housing that is slightly thicker than the distance between the tips of the rotating blades and the compressor housing. During initial operation of the compressor the rotating blades undergo a thermally induced expansion and cut a groove in the resin layer to a depth that corresponds to the clearance between the edges of the blades and the housing. To ensure continued operation of the compressor at maximum efficiency, once the initial groove is formed there should be no erosion of material from the resin layer other than that resulting from contact between this layer and the rotating blades. During operation of turbine-type aircraft engines various materials, including abrasives such as sand, grit and other types of debris, water, and relatively large objects such as birds are drawn into the intake of the compressor stages. The physical properties of the cured material used to form the abradable seal should be such as to resist erosion resulting from the impact of these foreign materials.
To achieve the physical properties desired for abradable seals various types of reinforcing and non-reinforcing fillers have been incorporated into the material used to form the seal. Reinforcing fillers are typically finely divided forms of precipitated and fumed types of silica. These fillers can be used in combination with non-reinforcing fillers such as quartz, calcium carbonate, talc and microspheres formed from glass or organic resins.
A shortcoming of prior art organosiloxane resin and elastomer compositions, including those described in the prior art as suitable for forming abradable seals for turbine type compressors on aircraft, is the tendency of some of these compositions, particularly those containing finely divided silica as the reinforcing filler, to erode relatively rapidly during operation.
The objective of this invention is to improve the machinability and increase the erosion resistance of cured organosiloxane compositions used as abradable seals without adversely affecting other desirable properties of the cured material.